NOx contained in exhaust gas has conventionally been removed by, for example, a method in which NOx is oxidized and then absorbed in alkaline solution or a method in which NOx is reduced to nitrogen by using a reducing agent such as ammonia, hydrogen, carbon monoxide or hydrocarbons. However, these conventional methods have their own disadvantages.
That is, the former method requires a means for handling the resulting alkaline waste liquid to prevent environmental pollution. The latter method, for example, when it uses ammonia as a reducing agent, involves a problem that ammonia reacts with SOx in exhaust gas to form salts, resulting in deterioration in catalytic activity at low temperatures. Accordingly, when NOx from moving sources such as automobiles is to be treated, the safety is a question.
On the other hand, when hydrogen, carbon monoxide or hydrocarbons is used as a reducing agent, the reducing agent reacts preferentially with oxygen since exhaust gas contains oxygen in a higher concentration than NOx. This means that substantial reduction of NOx needs a large quantity of a reducing agent, and hence resulting in remarkable fall of fuel efficiency.
It has been therefore proposed to catalytically decompose NOx in the absence of a reducing agent. However, the catalysts that have been conventionally known for direct decomposition of NOx have not yet been put to practical use due to their low decomposition activity for NOx. On the other hand, a variety of zeolites have been proposed as a catalyst for catalytic reduction of NOx using a hydrocarbon or an oxygen-containing organic compound as a reducing agent. In particular, Cu-ion exchanged ZSM-5 or H type (hydrogen type or acid type) zeolite ZSM-5 (SiO2/Al2O3 molar ratio=30 to 40) has been regarded as optimal. However, it has been found that even the H type zeolite has no sufficient reduction activity, and particularly the zeolite catalyst is rapidly deactivated on account of dealumination of the zeolite structure when water is contained in exhaust gas.
Under these circumstances, it has been necessary to develop a more active catalyst for the catalytic reduction of NOx. Accordingly, a catalyst composed of an inorganic oxide carrier material having silver or silver oxide supported thereon has recently been proposed, as described in EP-A1-526099 or EP-A1-679427. However, it has been found that the catalyst has a high activity for oxidation, but a low activity for selective reduction of NOx, so that the catalyst has a low conversion rate of nitrogen oxides to nitrogen. In addition, the catalyst involves a problem that it is deactivated rapidly in the presence of sulfur oxides. The catalyst catalyzes the selective reduction of NOx with hydrocarbons under full lean conditions, but it has a lower NOx conversion and a more narrow temperature window (temperature range) than the known three way catalyst. This makes it difficult for such lean NOx catalysts to be practically used. Thus, there has been a demand for developing a more heat-resistant and more active catalyst for catalytic reduction of nitrogen oxides.
In order to overcome the above-mentioned problems, a NOx storage-reduction system has recently been proposed as one of the most promising methods, as described in WO 93/7363 or WO 93/8383. In the proposed system, fuel is periodically spiked for a short moment to a combustion chamber in excess of the stoichiometric amount under rich conditions. Vehicles with lean burn engines can be driven at lower fuel consumption rates than conventional vehicles. It is because such vehicles can be driven at a much lower fuel/air ratio than the conventional vehicles. This NOx storage-reduction system for lean burn engines reduces NOx in two periodic steps at intervals of one to two minutes.
That is, in the first step, NO is oxidized to NO2 on platinum or rhodium catalyst under normal lean conditions, and the NO2 is absorbed as a nitrate such as potassium nitrate in an absorbent such an alkali compound as potassium carbonate or barium carbonate. Subsequently, rich conditions are formed for the second step, and are maintained for several seconds. Under the rich conditions, the absorbed (or stored) NO2 is released from the absorbent and is efficiently reduced to nitrogen with hydrocarbons, carbon monoxide or hydrogen on the platinum or rhodium catalyst. This NOx storage-reduction system works well over a long period of time in the absence of SOx. However, there is a problem that in the presence of SOx, the catalytic system deteriorates drastically due to the irreversible absorption of SOx at NO2 absorption sites on the alkali compound under either the lean or the rich conditions. In addition, since NOx is absorbed as a nitrate in the method, it is necessary that the rich conditions are strengthened to decompose and reduce the nitrate under the rich conditions, and consequently the method has a problem that fuel efficiency is deteriorated.
Accordingly, for the purpose of remedying the weak point or solving the problem in that the NOx storage-reduction system deteriorates in performance in the presence of SOx, there has been recently proposed in WO 02/8997 such a catalyst that has a purification ability close to the NOx storage-reduction system and a high SOx durability. The catalyst comprises:    (A) an outer catalyst layer comprising an outer catalyst component, wherein the outer catalyst component comprises
(a) ceria or;
(b) praseodymium oxide or;
(c) at least one selected from the group consisting of a mixture of oxides of at least two elements and a composite oxide of at least two elements, the elements being selected from the group consisting of cerium, zirconium, praseodymium, neodymium, gadolinium and lanthanum; and    (B) an inner catalyst layer comprising an inner catalyst component, wherein the inner catalyst component comprises
(d) at least one selected from the group consisting of platinum, rhodium, palladium and oxides thereof; and
(e) a carrier.
Further, there has been proposed in WO 02/22255 a catalyst that has a high SOx durability, which comprises an outer catalyst layer comprising a first catalyst component selected from rhodium, palladium and oxides thereof and a second catalyst component selected from zirconia, cerium oxide, praseodymium oxide, neodymium oxide and mixtures thereof, and an inner catalyst layer comprising a third catalyst component selected from rhodium, palladium, platinum and oxides thereof.